My tenants had non-certified work done

It leaves you with insurance issues on the house unfortunately. Get all the dodgy work taken out/made safe make sure the EICR is satisfactory, and maybe get your tenants to pay for some/all of it.

It leaves you in a situation where your tenant has knowingly caused criminal damage to your property. In the interests of safety and compliance you are best to have them removed from the property and reinstated correctly should you wish to allow the tenant to keep the sockets. In terms of financial liability it would be reasonable to expect to the tenant to cover the costs of making good the damage done. If you have a letting agent or a solicitor it would be best to ask them how to approach this. You may find it easier to charge them via invoice rather than to argue about the deposit.

This is a common occurrence and it’s also the reason why it is essential to have regular EICR’s completed. Unfortunately it now leaves you as the landlord responsible for rectifying the work. It sounds as though the work wouldn’t be anything close to complying with the current regs so it would be best to remove the sub standard work and try and get the tenant to foot the bill.

Just get the dodgy bit removed, it sounds a simple enough job to just disconnect. The tenants shouldn't have done any DIY electrics, if they want it reinstated tell them they will have to pay for an electrician to do the job properly and get certification. Make sure you have any C1, C2 or FI corrective work identified by the EICR completed, and get the relevant certification or compliance notes to verify the Installation is safe for continued use.